Patent GB-1070810-A (GB)
Title: Improvements in or relating to automatic protecting circuit arrangements for radio and like transmitters
Inventor: DOWNING ALBERT CHARLES
IPC: H03F1/52
Filed: 1965-01-12
Abstract: 1,070,810. Voltage measurements. MARCONI CO. Ltd. Dec. 16, 1965 [Jan. 12, 1965], No.1431/65. Heading G1U. [Also in Divisions G3 and H4] An arrangement for protecting two parallel connected radio or like transmitters comprises means for measuring the carrier level of modulated carrier signals derived from each transmitter against a reference level, means for comparing the modulation depths in modulated carrier signals derived from the two transmitters, and means for producing automatic safety action if either the carrier level falls below said reference level or if the difference between the compared modulation depths exceeds a predetermined amount. As shown in Fig. 1, outputs from the two transmitters (not shown) are supplied via transformers 2 to identical circuits A and B respectively. Considering circuit A, the transformer 2 supplies a rectifier bridge 3 which produces at potentiometer 4 a negative D.C. potential proportional to the carrier level and at potentiometer 5 an audio-frequency potential proportional to the modulation depth. The D.C. potential is applied to the base of a transistor 6 and so long as this potential exceeds a reference potential determined by Zener diode 7 the transistor 6 conducts. This holds transistor 8 non-conductive, providing no other voltage is applied to its base, and causes transistor 9 to conduct, so that a relay 10 is operated and switch 11 controlling a warning light 12 is open. When the carrier level drops below a predetermined amount adjustable by potentiometer 4, the lamp lights, Capacitor 13 connected across the relay coil provides a delay so that the alarm is not operated in response to a momentary drop in carrier level. The A.F. potential is fed via transformer 14 to a push-pull amplifier comprising transistors 15, 16, 17 and 15', 16', 17' arranged to have a high input impedance and provided with potentiometer 18 for balancing. Capacitor 19 eliminates 'hum' effects. Two equal outputs are taken via transformer 20 to rectifier bridges 21, 22, the rectified output from bridge 22 in circuit A being combined in opposition to the output from bridge 21 of circuit B in transistor 25 of circuit B, and the outputs of bridges 21, 22 of circuits A and B respectively being similarly combined in transistor 25 of circuit A. The resistors 26, 27 are dimensioned so that normally the current through the former is less than through the latter the difference between these currents causing diode 28 to conduct and cut off transistor 25. If the output of bridge 21 falls below a certain amount the current through resistor 26 will exceed that through resistor 27 and transistor 25 will conduct and cut off normally conductive transistor 30 causing transistor 8 to conduct and the warning lamp 12 to light as described previously. Separate relays may be provided for the two types of fault. The relay may be arranged to switch off the power supply in the event of a fault. The circuit of Fig. 1 requires that continuous modulation be present during the delay caused by capacitor 13 and to overcome this the circuit between transistors 25 and 8 is modified, Fig. 2 (not shown) by the inclusion of a bi-stable circuit which remains operated once a modulation fault occurs, until reset by a push button, so that an alarm is given whether or not modulation is present subsequently. A filter suppressing transients also is included in this part of the circuit.